1. Field of the Invention
The present invention relates to a semiconductor device having N or less pieces of semiconductor chips, more particularly to power semiconductor chips, mounted on a common substrate capable of mounting N pieces of semiconductor chips.
2. Description of the Related Art
First, reference will be made to FIG. 9 for the description of a sealed semiconductor device 30 of the prior art. The semiconductor device 30 comprises four power semiconductor chips 2 mounted on a lead frame 25 which is a common substrate, and sealed together by means of a molding resin 5. Each of the four semiconductor chips has electrodes on the front and back sides thereof. Hereinafter in this specification, an electrode formed on a surface of the semiconductor chip which makes contact with the common substrate will be referred to as back electrode (first electrode) and an electrode formed on a surface of the semiconductor chip opposite to the contact surface will be referred to as front electrode (second electrode). The back electrode of each semiconductor chip 2 is electrically connected to a back electrode lead 3c via a wiring pattern formed on the surface of the lead frame 25, and the front electrode is connected to a front electrode lead 3d via a wire bonding 4. The back electrode lead 3c and the front electrode lead 3d are sealed together with the molding resin 5, but are electrically insulated from each other.
The semiconductor device 30 of the prior art has such problems as described below. That is, the number of semiconductor chips 2 which can be mounted on the semiconductor devices 30 varies depending on the application of the semiconductor device 30. For example, besides the semiconductor device having four semiconductor chips 2 mounted thereon as shown in FIG. 9, there is a semiconductor device having three power semiconductor chips mounted on a lead frame. Because the lead frame of such a semiconductor device is designed in accordance to the number of semiconductor chips mounted thereon, it is necessary to design the lead frame of the semiconductor device having three semiconductor chips 2 mounted thereon and the lead frame of the semiconductor device having four semiconductor chips 2 mounted thereon separately, there has been a problem of a high development cost. Further, when producing the semiconductor device having three semiconductor chips 2 mounted thereon and the semiconductor device having four semiconductor chips 2 mounted thereon, it is necessary to change the production apparatuses used in the production line in accordance to the lead frame dimensions, and therefore different production lines must be prepared for the semiconductor device having three semiconductor chips 2 mounted thereon and for the semiconductor device having four semiconductor chips 2 mounted thereon. This leads to increased production cost for the semiconductor device.
Further, in the case of the semiconductor device 30 is used with a high voltage at a high temperature, the semiconductor device 30 needs to be connected to an external heat sink. Since the semiconductor chip 2 has the electrodes provided on the front and back sides thereof and the lead frame 25 is electrically conductive, the back electrode of the semiconductor chip 2 needs to be insulated from an external heat sink. In the prior art, the external heat sink and the lead frame 25 are put into contact via an insulation layer and the semiconductor device 30 is connected to the external heat sink with the lead frame 25 being fastened onto the external heat sink with a screw. Therefore the lead frame 25 has a screw hole 6 formed therein. At present, in order to produce the semiconductor device at a lower cost, there is a demand for eliminating the insulation film provided between the lead frame and the external heat sink thereby reducing the material cost.